Functions and control flow

A function names a task you want to reuse. Give parameters types and put the result type after ->. Return a value with return. A function without a result can omit -> Unit.

Save this as control_flow.xen:

fn doubled(value: Int) -> Int {
    return value * 2;
}

fn main() {
    let mut total = 0;
    for number in 1..5 {
        total = total + doubled(number);
    }
    let label = if total >= 20 { "enough" } else { "more" };
    println(total);
    println(label);
    let mut remaining = 2;
    while remaining > 0 {
        println(remaining);
        remaining = remaining - 1;
    }
}

$ xen run control_flow.xen
20
enough
2
1

Choose with a condition

In if condition { ... } else { ... }, the condition must be a Bool. An if used as a value requires else and must produce the same type in every branch. As with label above, leave the branch’s final value without a semicolon.

When selecting actions, each branch can contain several statements. Use else if to check another condition.

Repeat work

1..5 produces 1 through 4; the end is excluded. A for binding is visible only in its body. A range whose start is at least its end is empty. There is no syntax for range steps or reverse ranges.

while repeats while its condition is true. Make sure the body can change the condition. break; ends the innermost loop; continue; skips the rest of the current iteration. The next chapter covers collection iteration.

Return values and function values

A function with a result must return on every path. An ordinary function does not return its last expression automatically; write return. Functions can call themselves. At most six parameters are supported. Recursive calls have a depth limit, so consider a loop for long inputs.

A concrete named function can be stored or passed using a type such as fn(Int) -> Int. In the program above, let transform: fn(Int) -> Int = doubled; followed by transform(3) produces 6. Anonymous functions and closures are unavailable.

Try a change

Change the range to 1..6 and observe the total. Try adding continue to skip one number.